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Full text of "Alexander von Humboldt"

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178                              LIFE  OF
rocks lying deep in the mines., the glowing mass
which Tolcanos eject from the depth of the earth.
Hnroboldt does not venture to fix the boundary of
the hardened surface and the fluid centre; but he
considers that even in these more fluid parts, the
movements of ebb and tide dependent on the sun
and moon prevail. Now, as experience shows us that
heat, in a perpendicular line downwards of 92 Parisian
feet, increases by one degree of the decennial Celsius
thermometer, granite must exist in a fluid state at a
depth of 5*2 geographical miles, four or five times as
much as the height of the highest point of the
Himalaya mountains.
Humboldt distinguishes three modes of the deve-
lopment of the inner heat of the earth. The first is,
that the strata of earth are periodically wanned
and cooled by the sun and the seasons of the year,
and thus arises a stream of heat from the exterior
inwards, and then again from the interior outwards,*
Secondly, in the regions of the equator, a portion of
heat penetrates the earth, and flows in it towards the
cooler poles,, where it is again united with the air.
Finally, our earth has been, since incalculable ages,
in a state of gradual refrigeration ; the inner central
heat, which originally made the earth glowing hot,
loses more and more by the gradual discharge towards
the surface and into the atmosphere, although millen-
niums do not suffice to measure the degrees. We
therefore live, as Humboldt expresses himself, between
the glowing heat of the lower strata and the cold
atmosphere, of which the temperature is probably
"below the freezing-point of quicksilver. (40 degrees
of cold of Celsius = 32 degrees of Reaumur.)
There are celebrated naturalists who have denied
the uninterrupted increase of heat from the surface to
* Tliis heat does not penetrate far. In the temperate zones the
strata of permanent temperature begins at a depth of fifty-five to
sizty feet ; and at half the depth winter and summer warmth have
scarcely half a degree influence 011 the Thermometer. In the tropics,
the imchangeaHe temperature Ees one 'foot below tfee surface.